Before this , I was a Windows user and some of my previous documents were saved in .docx format.

I tried to use LibreOffice to open those .docx file and I can view all text, however I can't view the flowchart I drew and also mathematical equations.

Another issue is, if I create new flowchart with LibreOffice and save it in .docx file, when I re-open that file, I can't view those flowcharts, but those flowcharts are there, occupied space. No problem for .odt format of course.

Because Microsoft does not publish standard documents for his document formats and does not use the formats all the other systems use. Microsoft does this to annoy users and make them think open source software were bad.
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Michael KDec 14 '11 at 11:35

Basically what Michael K said. Microsoft does not publish specs for their format - office suites must guess at the content of .docx files. This is done so that Microsoft keeps the monopoly on writing suites and office products - they can say things like "Only Office has complete compatability with .docx files".

As a result of this, independent suites such as LibreOffice and OpenOffice can't keep up with the newest features completely. There's unfortunately no way around this; LibreOffice Writer just doesn't know what to do with that part of the document.

As a workaround, you can use a software program called "PlayOnLinux" (it's in the official repositories, just open software center) to install Office in Ubuntu. Currently it supports Excel, Powerpoint, Outlook, and Word from all versions of Office. You just need to have the ISO or disc lying around along with a serial and the wizard will guide you through it step-by-step.

To install PlayOnLinux, either look in the Software Center or type sudo apt-get install playonlinux in the terminal.